Urophora cardui

Urophora affinis

Urophora cardui ...induces a large spherical or irregular multichambered gall in the stems of Cirsium arvense (L.) Scop. The fly requires moist sites ... 

[In] June ... [e]ggs are laid into an apical leaf bud ... The larvae tunnel down the stem until they reach the zone of vascular differentiation where they induce the gall. The tunnel, which expands as the gall grows, fills with callus ... Larvae ... overwinter in the gall, remaining dormant as long as the gall is intact, but pupating when exposed to air. In nature this occurs in the spring ... Final gall size is a function of the number of larvae in it ... The gall ... is soft and fleshy while growing, but hard when mature ... [G]all parenchyma beyond the nutritive layer becomes lignified ... 

[B&W photo on pg. 15]

Urophora affinis is a tephritid fly that forms a unilocular receptacle gall (Fig. 16) in [plants belonging to a] Centaurea subgenus [that] contains the weeds C. diffusa and C. [stoebe], diffuse and spotted knapweed. The fly is native to Europe ... Freshly hatched larvae chew down a floret, consume the ovule, and then feed on the inner layer of the ovary wall. After about 12 days, they penetrate the base of the ovary to feed on the receptacle. Cells of the ovary wall and receptacle proliferate to enclose the larva in a thick layer of parenchyma. Vascular bundles extend from the receptacle into this layer ... [After] about 25 days ... a sclerenchyma layer forms around the gall except for the apex ... 
The corolla is suppressed or absent in galled capitula and the stomata in the bracts are not enlarged ... The florets tunnelled by larvae of U. affinis are destroyed and adjacent ones abort ... 

Effectiveness of gall inducers in weed biological control